1. Field of the Invention
The present invention relates to a vibration measuring method and apparatus, and in particular, to a vibration measuring method and apparatus for measuring a vibrating state of a object using a self-mixing laser Doppler vibrometer.
The present invention is applicable to various fields associated with vibration analysis. Specific applications include engine vibration analysis, body propagating vibration analysis, interior-noise analysis, and muffler vibration analysis. Other manufacturing fields have various applications. That is, the present invention can be used for maintenance such as detection of vibration in a plant using a motor or diagnosis for leakage from a water or gas pipe. Furthermore, it can be applied to agricultural fields in, for example, determining the sugar content of large fruits such as watermelons using tapping sound.
The present invention enables measurement of a vibration frequency of a very small amplitude of, for example, 200 nm or changes in the speed of a vibrating surface and is thus applicable to inspection and calibration devices for vibration generating apparatuses or to abnormal-vibration detecting devices for long-time operating power apparatuses. Such inspection and calibration devices can be used, for example, to inspect quartz or ultrasonic oscillators for frequency or to calibrate function generators. In addition, such abnormal-vibration detecting devices can be used to detect defects in semiconductor fabricating apparatuses using high-frequency vibration, the defects resulting from a failure to efficiently propagate vibration energy by resonance, or these devices can be used to detect damage to tools such as drills.
Thus, the term xe2x80x9cmeasured objectxe2x80x9d, as used below, refers to an object the vibration of which is to be measured, the object ranging from an engine to a tool.
2. Description of the Prior Art
Conventional means for measuring the frequency of a vibrating object in a non-contact manner include a method for determining the frequency using, for example, a laser displacement gauge to which triangulation is applied. Additionally, a vibration measuring apparatus using a laser Doppler vibrometer and provided by the applicant is disclosed in Japanese Patent Laid-Open No. 11-287699. The approach described in this publication comprises detecting beat waves based on a difference between the frequency of an emitted light and the frequency of returning light having a Doppler frequency superposed thereon depending on the speed of a measured object, and detecting the displacement of the measured object or a change in the speed thereof based on the beat waves.
The approach described in this publication measures the displacement based on the number of beat waves, utilizing the fact that one beat wave is generated each time the measured object is displaced by a length (xcex/2) corresponding to the half of an oscillating wavelength xcex of the laser. Then, a reversing position of the progress direction is determined utilizing the fact that a beat wave corresponding to the reversing position has a deviating waveform and a large wavelength because the measured object is displaced by a length smaller than xcex/2.
In this conventional example, however, the accuracy of vibration measurements depends on the oscillating wavelength of a laser, so that it may be disadvantageously difficult to appropriately measure a displacement smaller than xcex/2 or conditions of fine high-frequency vibration having a vibration wavelength smaller than xcex/2. In addition, various signal processes are required to determine the reversing position, disadvantageously resulting in an enormous number of processes to be executed if the measured object has a high frequency. Further, if the measured object generates vibration comprising a plurality of vibrations complicatedly associated with one another instead of simple harmonic oscillation, so that it may be disadvantageously difficult to accurately calculate vibration conditions depending on the approach to determine the reversing position.
It is an object of the present invention to eliminate the disadvantages of the conventional example in order to provide a vibration measuring apparatus and method that enables vibration conditions of a measured object to be accurately measured without depending on a laser oscillating wavelength.
According to the present invention, an apparatus comprises a laser resonator for oscillating a laser beam and generating beat waves through self-mixture of a returning beam obtained when the oscillated emitted beam is reflected by the measured object with a beam oscillated and emitted when the returning beam is received, a modulated beat frequency generation controller for providing the beat waves with a modulated beat frequency that is higher than a Doppler frequency that is superposed on the returning beam while varying depending on a speed of the measured object, and a signal processor for executing signal processing on the beat waves generated by the self-mixture in the laser resonator and outputting a result of the processing as vibration information. The signal processor comprises a vibration information generator for generating vibration information from the beat waves by defining variations in the frequency of the beat waves as variations from a modulated beat frequency by the amount of a Doppler frequency. This configuration attains the above described object.
In the laser resonator, the returning beam obtained by shifting the emitted beam by an amount corresponding to the Doppler frequency depending on the speed of the measured object is self-mixed with the beam emitted when the returning beam is received, thereby generating the beat waves. At this time, the modulated beat frequency generation controller provides the beat waves with the modulated beat frequency that is higher than the Doppler frequency that is superposed on the returning beam while varying depending on the speed of the measured object. The modulated beat frequency generation controller may be adapted, for example., to provide the laser resonator with a laser driving current that varies an oscillating frequency from the laser resonator or to physically displace the laser resonator at a constant speed to generate as a modulated beat frequency a Doppler frequency that does not correspond to a movement speed that depends on the speed of the measured object.
Since the modulated beat frequency generation controller (or laser drive controller) provides the modulated beat frequency for the beat waves obtained by the self-mixture, the frequency of the beat waves (a Doppler frequency or an actually observed frequency of the beat waves) is the sum of the modulated beat frequency and the Doppler frequency that depends on the speed of the measured object. If the measured object is vibrating, since the Doppler frequency varies depending on the speed of the measured object, a beat wave corresponding to a position where the measured object reverses its moving direction has a zero Doppler frequency. At this time, the Doppler beat frequency (observed frequency) is the modulated beat frequency generated by the modulated beat frequency generation controller. Since the observed frequency of the beat wave at the reversing position is the modulated beat frequency, the beat wave at the reversing position does not deviate from its appropriate waveform.
As the measured object reverses its moving direction at the reversing position and then accelerates, the Doppler frequency varies. With a sine vibration, the speed is highest at a central position of the vibration. Accordingly, the Doppler beat frequency varies relative to the modulated beat frequency depending on variations in Doppler frequency. Thus, the beat waves are considered to be a signal obtained by means of frequency modulation using the Doppler frequency, the signal using the modulated beat frequency as a carrier frequency. Small changes in Doppler frequency can be determined by increasing the modulated beat frequency used as the carrier frequency; this means that accuracy is set irrespective of the oscillating wavelength of the laser resonator. If, for example, vibration of a measured object vibrating at a certain frequency is measured and if the modulated beat frequency is ten times as high as the vibration frequency, 10 periods of beat waves are obtained per period of the measured object, resulting in 10 pieces of Doppler frequency information.
The signal processor generates vibration information based on the beat waves. The beat waves themselves are useful as vibration information because they are a signal with a frequency modulated depending on the speed of the measured object. Additionally, a waveform obtained by subjecting the beat waves to frequency voltage conversion is a variation of the waveform of the Doppler frequency relative to the modulated beat frequency, which corresponds to a speed variation waveform. The speed variation waveform can be differentiated to obtain an acceleration variation waveform or integrated to obtain a displacement variation waveform, and the period of the speed variation waveform corresponds to a vibration period. Further, the modulated beat frequency is subtracted from the Doppler beat frequency to obtain the Doppler frequency. Then, the speed can be determined from the Doppler frequency and laser oscillating wavelength.